Multilayer Piezoelectric Device and Method for Manufacturing the Same

ABSTRACT

A multilayer piezoelectric device including a body having internal electrode layers and piezoelectric ceramic layers alternately stacked. The internal electrode layers contain Cu as a major component, the piezoelectric ceramic layers contain a compound oxide represented by Pb(Ti, Zr)O 3  as a major component, and a metal oxide (Nb 2 O 5 , Sb 2 O 5 , Ta 2 O 5 , or WO 3 ) containing Nb, Sb, Ta, or W, which is at least one of a pentavalent metal element and a hexavalent metal element, is incorporated in the piezoelectric ceramic layers such that the concentration of the metal oxide decreases with distance from the internal electrode layers. Thereby, even in a case where internal electrodes contain Cu as a major component, it is possible to provide a multilayer piezoelectric device which can be obtained by low-temperature firing while ensuring a sufficient piezoelectric constant.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of application Ser. No.12/131,371, filed Jun. 2, 2008, which is a continuation of InternationalApplication No. PCT/JP2006/320398, filed Oct. 12, 2006, which claimspriority to Japanese Patent Application No. JP2005-354332, filed Dec. 8,2005, the entire contents of each of these applications beingincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a multilayer piezoelectric device, suchas a piezoelectric actuator, a piezoelectric resonator, or apiezoelectric filter, and a method for manufacturing the multilayerpiezoelectric device.

BACKGROUND OF THE INVENTION

PZT-based piezoelectric ceramics containing Pb(Ti, Zr)O₃ (hereinafterreferred to as “PZT”) as a major component are widely known as typicalpiezoelectric ceramics used for multilayer piezoelectric devices, suchas piezoelectric actuators, piezoelectric resonators, and piezoelectricfilters.

Furthermore, as multilayer piezoelectric devices including PZT-basedpiezoelectric ceramics, devices which are fabricated by alternatelystacking internal electrode layers composed of an Ag—Pd alloy andceramic layers composed of a PZT-based piezoelectric ceramic, followedby simultaneous firing, are often used. However, since Pd is expensive,research and development has been actively carried out on multilayerpiezoelectric devices in which inexpensive Cu is used as the materialfor internal electrodes.

For example, Patent Document 1 describes a multilayer piezoelectricdevice including internal electrode layers containing Cu as a majorcomponent, and a PZT-based piezoelectric ceramic.

Patent Document 1 discloses a multilayer piezoelectric device includingelectrodes containing Cu as a major component, and a piezoelectricceramic, in which the piezoelectric ceramic contains, as a majorcomponent, a perovskite oxide represented by general formula ABO₃, the Asite including Pb, the B site including Zr and Ti, wherein the B siteincludes an acceptor element which is a divalent metal element and adonor element which is a pentavalent metal element, and the relationship0.42<a/b<0.5 is satisfied, wherein a represents the total molar amountof the acceptor element, and b represents the total molar amount of thedonor element.

When the electrodes containing Cu as the major component and thepiezoelectric ceramic are simultaneously fired, Cu in the electrodesdiffuses into the piezoelectric ceramic and acts as a divalent acceptorelement in the state of Cu²⁺. Consequently, in the multilayerpiezoelectric device of Patent Document 1, in the B site of thepiezoelectric ceramic composition, the donor content is adjusted to bein excess so that the relationship 0.42<a/b<0.5 is satisfied, wherein arepresents the total molar amount of the acceptor element, and brepresents the total molar amount of the donor element. Thereby, thedecrease in the average valence in the B site due to the diffusion of Cuis cancelled out, enabling prevention of a decrease in the piezoelectricconstant.

Patent Document 1: International Publication No. WO2005/071769 pamphlet

SUMMARY OF THE INVENTION

In the multilayer piezoelectric device described in Patent Document 1,by adjusting the donor content to be in excess in the B site of thePZT-based piezoelectric ceramic, even when the internal electrodescontain Cu as the major component, it is possible to obtain apiezoelectric constant substantially equivalent to that in the casewhere internal electrodes are composed of an Ag—Pd alloy.

However, when the B site is set so that the donor content is in excess,sinterability decreases. Consequently, it is necessary to perform firingat relatively high temperatures. For example, in the multilayerpiezoelectric device of Patent Document 1, the firing temperature is setat 1,000° C. (refer to paragraph [0048], Patent Document 1).

On the other hand, the melting point of Cu is about 1,050° C. Since thefiring temperature of 1,000° C. is close to the melting point of Cu, itis desirable to further decrease the firing temperature. Moreover, ifthe firing temperature is decreased, it is considered that the amount ofdiffusion of Cu can be reduced. From such a standpoint, it is alsodesirable to decrease the firing temperature. Furthermore, if the firingtemperature is decreased, the energy required for firing can be reduced.Thus, fuel and electric power can be saved, and also the manufacturingcost can be reduced.

The present invention has been achieved under these circumstances. It isan object of the invention to provide a multilayer piezoelectric devicewhich can be obtained by low-temperature firing while ensuring asufficient piezoelectric constant even in the case where internalelectrodes contain Cu as a major component, and a method formanufacturing the multilayer piezoelectric device.

As a result of diligent studies conducted by the present inventors toachieve the object described above, it has been found that byincorporating a metal oxide in a piezoelectric ceramic such that theconcentration of the metal oxide decreases with distance from internalelectrode layers, it is possible to charge-compensate for the decreasein the average valence in the B site of the piezoelectric ceramiccomposition due to the diffusion of Cu, and thereby it is possible toobtain a satisfactory piezoelectric constant even if the donor contentis not adjusted to be in excess in the B site of the piezoelectricceramic. Moreover, since it is not necessary to set the donor content tobe in excess, sinterability can be improved, and a desired multilayerpiezoelectric device can be obtained by low-temperature firing.

That is, a multilayer piezoelectric device according to the presentinvention is characterized by including a body in which internalelectrode layers and piezoelectric ceramic layers are alternatelystacked, wherein the internal electrode layers contain Cu as a majorcomponent, the piezoelectric ceramic layers contain a compound oxiderepresented by Pb(Ti, Zr)O₃ as a major component, and a metal oxidecontaining at least one of a pentavalent metal element and a hexavalentmetal element is incorporated in the piezoelectric ceramic layers suchthat the concentration of the metal oxide decreases with distance fromthe internal electrode layers.

The multilayer piezoelectric device of the present invention is alsocharacterized in that the metal element is at least one selected fromthe group consisting of Nb, Sb, Ta, and W.

Furthermore, a method for manufacturing a multilayer piezoelectricdevice according to the present invention is characterized by includinga stacking step of alternately stacking internal electrode patterns andpiezoelectric ceramic green sheets to form a laminate, and a firing stepof firing the laminate to form a body in which internal electrode layersand piezoelectric ceramic layers are alternately stacked, wherein thepiezoelectric ceramic green sheets contain a compound oxide representedby Pb(Ti, Zr)O₃ as a major component; the internal electrode patternscontain a conductive powder containing Cu and at least one of apentavalent metal element and a hexavalent metal element; and in thefiring step, the metal element is diffused in the form of a metal oxidefrom the internal electrode layers into the piezoelectric ceramic layerssuch that the concentration of the metal oxide decreases with distancefrom the internal electrode layers.

The method for manufacturing the multilayer piezoelectric device of thepresent invention is also characterized in that the metal element is atleast one selected from the group consisting of Nb, Sb, Ta, and W.

The method for manufacturing the multilayer piezoelectric device of thepresent invention is also characterized in that the metal oxide isrepresented by any one of Nb₂O₅, Sb₂O₅, Ta₂O₅, and WO₃.

The method for manufacturing the multilayer piezoelectric device of thepresent invention is also characterized in that the content of the metaloxide is less than 40.0% by weight on the basis of the total content ofthe conductive powder and the metal oxide.

According to the multilayer piezoelectric device of the presentinvention, a metal oxide containing at least one of a pentavalent metalelement and a hexavalent metal element (Nb, Sb, Ta, W, or the like) isincorporated in the piezoelectric ceramic layers such that theconcentration of the metal oxide decreases with distance from theinternal electrode layers. Consequently, the decrease in the averagevalence due to the diffusion of Cu can be charge-compensated for by themetal oxide, and thereby it is possible to obtain a good piezoelectricconstant even if the donor content is not adjusted to be in excess toperform charge compensation. Furthermore, since it is not necessary toset the B site so that the donor content is in excess, sinterability canbe improved, and firing can be performed at lower temperatures.Moreover, in the vicinity of the internal electrodes where the decreasein the average valence due to the diffusion of Cu is large, chargecompensation can be effectively performed by the metal oxide. Meanwhile,in locations distant from the internal electrode layers where thedecrease in the average valence due to the diffusion of Cu is not verylarge, since the concentration of the metal oxide is low, the metaloxide is not diffused excessively into the piezoelectric ceramic layers.Thereby, it is also possible to suppress the decrease in sinterability,thus being favorable to low-temperature firing.

Furthermore, according to the method for manufacturing the multilayerpiezoelectric device of the present invention, the internal electrodepatterns contain a conductive powder containing Cu and at least one of apentavalent metal element and a hexavalent metal element (Nb, Sb, Ta, W,or the like), and in the firing step, the metal element is diffused, inthe form of a metal oxide, such as Nb₂O₅, Sb₂O₅, Ta₂O₅, or WO₃, from theinternal electrode layers into the piezoelectric ceramic layers suchthat the concentration decreases with distance from the internalelectrode layers. Consequently, a concentration gradient of the metaloxide is formed in the piezoelectric ceramic layers. Therefore, it ispossible to easily fabricate a multilayer piezoelectric device in whichthe concentration of a metal oxide decreases with distance from internalelectrode layers in piezoelectric layers, and it is possible tofabricate a multilayer piezoelectric device in which a satisfactorypiezoelectric constant can be obtained even if firing is performed atlow temperatures.

Furthermore, since the content of the metal oxide is less than 40.0% byweight on the basis of the total content of the conductive powder andthe metal oxide, the connectivity between the internal electrode layersand external electrodes is not decreased. Thus, it is possible to obtaina high-quality multilayer piezoelectric device with high reliability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view showing a multilayer piezoelectricdevice of the present invention.

FIG. 2 is a graph showing the relationship between the Nb₂O₅ content ininternal electrode patterns and the piezoelectric constant d₃₃.

FIG. 3 is a graph showing the relationship between the WO₃ content ininternal electrode patterns and the piezoelectric constant d₃₃.

REFERENCE NUMERALS

-   -   20 body    -   11 piezoelectric ceramic layer    -   21, 22 internal electrode layer

DETAILED DESCRIPTION OF THE INVENTION

Best modes for carrying out the present invention will now be describedwith reference to the accompanying drawings. FIG. 1 is a cross-sectionalview showing a multilayer piezoelectric device according to anembodiment of the present invention.

The multilayer piezoelectric device includes a body 10 in whichpiezoelectric ceramic layers 11 and internal electrode layers 21 and 22are alternately stacked, and external electrodes 31 and 32 disposed onthe surface of the body 10. The internal electrode layers 21 connectedto the external electrode 31 and the internal electrode layers 22connected to the other external electrode 32 are alternately arranged.In the multilayer piezoelectric device, when a voltage is appliedbetween the external electrodes 31 and 32, an electric field isgenerated between each pair of internal electrode layers 21 and 22, andthe piezoelectric ceramic layers 11 expand or contract.

The internal electrode layers 21 and 22 contain a conductive powdercontaining Cu as a major component, and a metal oxide containing atleast one of a pentavalent metal element and a hexavalent metal element.Here, as the pentavalent metal element, Nb, Sb, or Ta can be used, andas the hexavalent metal element, W (tungsten) can be used. Consequently,the metal oxide is contained, for example, in the form of Nb₂O₅, Sb₂O₅,Ta₂O₅, or WO₃, in the internal electrode layers 21 and 22.

As described above, the internal electrode layers 21 and 22 contain theconductive powder containing Cu as the major component, and preferably,the conductive powder contains Ni as a minor component. The meltingpoint of Cu is 1,050° C., while the melting point of Ni is high at1,450° C. Therefore, by incorporation of Ni, the melting point of theinternal electrode layers 21 and 22 is increased and the diffusion of Cuis inhibited. Moreover, since Ni is more easily oxidized than Cu, theoxidation of Cu is inhibited, and thus the diffusion of Cu is alsoinhibited. Furthermore, when Ni is contained as the minor component, theratio of Cu content to Ni content, in terms of weight, is preferably85:15 to 70:30. The reason for this is that when the Ni content is 15%by weight or more, the effect of inhibiting the oxidation and diffusionof Cu is high, and the decrease in the piezoelectric constant due to thediffusion of Cu can be effectively inhibited. However, if the Ni contentexceeds 30% by weight, the Cu content decreases excessively, and thereis a possibility that the internal electrode layers 21 and 22 may bedisconnected, which is unfavorable.

When Ni is contained in the internal electrode layers 21 and 22, Ni ismostly present as NiO. When Ni is incorporated in the internal electrodelayers 21 and 22, preferably, Ni is also incorporated in the compositionof the piezoelectric ceramic layers 11. The reason for this is that byincorporating Ni in the composition of the piezoelectric ceramic layers11, it is possible to inhibit NiO from diffusing from the internalelectrode layers 21 and 22 into the piezoelectric ceramic layers 11.

In addition to the case where the piezoelectric ceramic layers 11contain a PZT-based compound oxide having a perovskite structurerepresented by general formula ABO₃ as a major component and arecomposed of PZT(Pb(Ti, Zr)O₃) alone, the piezoelectric ceramic layers 11may have a composition in which another perovskite compound oxide, suchas Pb(Ni, Nb)O₃ or Pb(Zn, Nb)O₃, is allowed to form solid solutions withPZT, and thus the B site is partially replaced by Ni, Nb, Zn, or thelike.

Furthermore, the PZT-based compound oxide may have a composition inwhich the B site is partially replaced by any of various combinations ofcations, such as a combination of a monovalent cation and a pentavalentcation, a combination of a divalent cation and a pentavalent cation, acombination of a trivalent cation and a pentavalent cation, or acombination of a trivalent cation and a hexavalent cation. Here, as themonovalent cation, Na or K can be used, and as the divalent cation, Ni,Zn, Co, Mg, Mn, Fe, Cr, or Cu can be used. Furthermore, as the trivalentcation, Fe, In, Sc, or Yb can be used, and as the pentavalent cation,Nb, Sb, Ta, or V can be used. Furthermore, as the hexavalent cation, Wor the like can be used.

The average valence in the B site is preferably set at 4 or a value inthe vicinity of 4, and specifically in a range of 3.95 to 4.05. Thereason for this is that if the average valence in the B site is lessthan 3.95, there is a possibility that piezoelectricity may be decreaseddue to the excessive diffusion of CuO from the internal electrode side.On the other hand, if the average valence exceeds 4.05, there is apossibility that sinterability may be decreased, and it may becomedifficult to perform firing at low temperatures.

Furthermore, as necessary, Pb constituting the A site of the PZT-basedcompound oxide may be partially replaced by Ba, Sr, or Ca, which is adivalent cation, or La, Y, Bi, or Nd, which is a trivalent cation. Insuch a case, the replacement ratio by these elements is preferably 5mole percent or less. The reason for this is that if the replacementratio exceeds 5 mole percent, there is a possibility that sinterabilitymay be decreased.

The average valence in the A site is preferably set at 2 or a value inthe vicinity of 2, and specifically in a range of 1.94 to 2.05. Thereason for this is that if the average valence in the A site is lessthan 1.94 or more than 2.05, there is a possibility that sinterabilitymay be decreased, and it may become difficult to perform firing at lowtemperatures.

The piezoelectric ceramic layers 11 contain a metal oxide containing apentavalent or hexavalent metal element, and the concentration of themetal oxide is high in the vicinity of the internal electrode layers 21and 22 and decreases with distance from the internal electrode layers 21and 22. Such a structure is formed by allowing the metal oxide, such asNb₂O₅, Sb₂O₅, Ta₂O₅, or WO₃, contained in the internal electrode layers21 and 22 to diffuse from the internal electrodes 21 and 22 into thepiezoelectric ceramic layers 11 during the firing process. By settingthe concentration of the metal oxide containing a pentavalent orhexavalent metal element so as to decrease with distance from theinternal electrode layers 21 and 22 as described above, chargecompensation can be performed by the pentavalent or hexavalent metaloxide even if the content of the donor element in the piezoelectricceramic layers 11 is not adjusted to be in stoichiometric excess (donorcontent being in excess) to compensate for the decrease in the averagevalence in the B site due to the diffusion of Cu.

By charge-compensating for the decrease in the average valence of the Bsite due to the diffusion of Cu by use of the metal oxide as describedabove, it is not necessary to perform charge compensation by setting thedonor content to be in excess, and it is possible to obtain a multilayerpiezoelectric device having a sufficient piezoelectric constant bylow-temperature firing. Moreover, in the vicinity of the internalelectrodes 21 and 22 where the decrease in the average valence due tothe diffusion of Cu is large, charge compensation can be effectivelyperformed. Meanwhile, in locations distant from the internal electrodelayers 21 and 22 where the decrease in the average valence due to thediffusion of Cu is not very large, since the pentavalent or hexavalentmetal oxide is not present in an amount more than necessary, it is notnecessary to increase the content of the pentavalent or hexavalent metaloxide to a level more than necessary. From this standpoint, it is alsopossible to suppress the decrease in sinterability, and low-temperaturefiring can be performed.

Furthermore, the present invention does not exclude the case where thecomposition of piezoelectric ceramic layers is arranged so that thedonor content is in excess. That is, even in the case where thecomposition of piezoelectric ceramic layers is arranged so that thedonor content is in excess, by applying the present invention, it ispossible to compensate for the decrease in the average valence due tothe diffusion of Cu without setting the donor content to be markedly inexcess.

The external electrodes 31 and 32 are composed of Cu, an Ag—Pd alloy, orthe like, and are formed on the surface of the body 10 by baking or thelike.

A method for manufacturing the multilayer piezoelectric device will nowbe described.

First, starting materials, such as Pb₃O₄, TiO₂, ZiO₂, and according toneed, NiO, ZnO, Nb₂O₅, etc., are prepared and mixed at a predeterminedratio. The resulting mixture is pulverized, and then calcined to obtaincalcined powder of a piezoelectric ceramic. The calcined powder iskneaded with a binder and a plasticizer, and piezoelectric ceramic greensheets are formed by a doctor blade process.

Furthermore, a conductive paste for internal electrodes is prepared, theconductive paste containing Cu powder, a metal oxide containing apentavalent or hexavalent metal element, specifically, for example, atleast one of Nb₂O₅, Sb₂O₅, Ta₂O₅, and WO₃, and according to need, Nipowder. When Ni powder is incorporated in the conductive paste, theratio of Cu powder content to Ni powder content, in terms of weight, ispreferably set at 70:30 to 85:15. Furthermore, preferably, the contentof the metal oxide is set to be less than 40.0% by weight on the basisof the total content of the conductive powder and the metal oxide(hereinafter, both of these are referred to as the “total solid”). Thereason for this is that if the content of the metal oxide exceeds 40.0%by weight on the basis of the total solid content, the Cu content in theinternal electrode layers 21 and 22 decreases, and therefore, there is apossibility that poor connection may occur between the internalelectrodes 21 and 22 and the external electrodes 31 and 32.

Next, the conductive paste is printed on the piezoelectric ceramic greensheets to form internal electrode patterns. Then, piezoelectric ceramicgreen sheets provided with the internal electrode patterns and blankpiezoelectric ceramic green sheets not provided with the internalelectrode patterns are stacked in a predetermined order to form alaminate.

Next, the laminate is subjected to a firing process at a firingtemperature of 950° C. to 1,000° C. for about 5 to 10 hours to form abody 10. From the standpoint that the oxidation of Cu which is the majorcomponent of the internal electrode layers 21 and 22 is inhibited andthe reduction of Pb which is contained in the piezoelectric ceramiclayers 11 is inhibited, the firing atmosphere is preferably set to be anatmosphere having an oxygen partial pressure between the equilibriumpartial oxygen pressure for Pb—PbO and the equilibrium partial oxygenpressure for Cu—CuO.

The pentavalent or hexavalent metal oxide contained in the internalelectrode patterns diffuses into the piezoelectric ceramic layers 11such that the concentration decreases with distance from the internalelectrode layers 21 and 22 during the firing process. Thereby, thedecrease in the average valence in the B site due to the diffusion of Cufrom the internal electrode layers into the piezoelectric ceramic layersduring the firing process is charge-compensated for, and it is possibleto suppress the degradation of piezoelectric ceramic properties even ifthe B site is not set so that the donor content is in excess.

Next, by baking conductive paste containing Cu or an Ag—Pd alloy as themajor component onto the surface of the resulting body, externalelectrodes are formed. Polarization is performed by applying apredetermined voltage to the body in an oil, and thereby a multilayerpiezoelectric device is fabricated.

In this embodiment, as described above, since the metal oxide isdiffused from the internal electrode layers 21 and 22 into thepiezoelectric ceramic layers 11, it is not necessary to set thecomposition of the piezoelectric ceramic layers 11 so that the donorcontent is in excess, and low-temperature firing can be performed.

The present invention is not limited to the embodiment described above.In the embodiment described above, the metal oxide containing apentavalent or hexavalent metal is incorporated in the internalelectrode patterns. As long as the metal element is distributed in theform of the metal oxide in the piezoelectric ceramic layers 11, themetal element is not necessarily contained in the form of the metaloxide in the internal electrode patterns. That is, in the internalelectrode patterns, the metal element may be contained, for example, asan elemental metal, or as a compound, such as a carbonate, a hydroxide,or an organic compound. Furthermore, when a pentavalent or hexavalentmetal is incorporated in a form other than the metal oxide in theconductive paste for internal electrodes, the amount of addition ispreferably set to be less than 40.0% by weight in terms of the metaloxide.

Furthermore, in the present invention, it is possible to obtain asufficient piezoelectric constant by low-temperature firing withoutsetting the composition of the B site of the piezoelectric ceramic sothat the donor content is in excess. However, the present invention doesnot exclude the case where the composition of the B site is set so thatthe donor content is in excess. It is acceptable to set the donorcontent to be in excess within a range that does not affect thecharacteristics and does not decrease sinterability.

Furthermore, the metal oxide that diffuses from the internal electrodelayers into the piezoelectric ceramic layers may be present in grainboundaries or at crystal triple points in the piezoelectric ceramiclayers, or may form solid solutions with the compound oxide having theperovskite structure and may be present in crystal grains in thepiezoelectric ceramic layers.

Examples of the present invention will be specifically described below.

Example 1

First, as starting materials for a piezoelectric ceramic, Pb₃O₄, TiO₂,ZiO₂, NiO, ZnO, and Nb₂O₅ powders are prepared and weighed so as tosatisfy the composition shown in compositional formula (1) below.

Pb{(Ni_(1/3)Nb_(2/3))_(0.1)(Zn_(1/3)Nb_(2/3))_(0.1)Ti_(0.42)Zr_(0.38)}O₃  (1)

The weighed starting materials were mixed and pulverized for 16 hours,and then calcined at 880° C. to obtain calcined powder of thepiezoelectric ceramic. The calcined powder was kneaded with a binder anda plasticizer, and piezoelectric ceramic green sheets with a thicknessof 120 μm were obtained by a doctor blade process.

Next, as conductive powders, a Cu powder and a Ni powder were prepared,and as a metal oxide containing a pentavalent metal element, Nb₂O₅ wasprepared.

Then, the Cu powder, the Ni powder, and Nb₂O₅ were weighed so that theratio of the Cu powder content to the Ni powder content, in terms ofweight, was 85:15 and the Nb₂O₅ content was 0% to 40% by weight on thebasis of the total solid content (Nb₂O₅, Cu powder, and Ni powder). Abinder, etc. was added to the weighed materials, and the mixture waskneaded in an organic vehicle to form a conductive paste for internalelectrodes.

Next, the conductive paste was screen-printed on the piezoelectricceramic green sheets to form internal electrode patterns. Then,piezoelectric ceramic green sheets provided with the internal electrodepatterns and blank piezoelectric ceramic green sheets not provided withthe internal electrode patterns were stacked in a predetermined order,followed by pressure bonding using a press. Thereby, a laminateincluding 80 internal electrode pattern layers was formed.

The laminate was subjected to a debinding process, and then firing wasperformed for 5 hours in an atmosphere having an oxygen partial pressurebetween the equilibrium partial oxygen pressure for Pb—PbO and theequilibrium partial oxygen pressure for Cu—CuO to form a body. Thefiring temperature was set at 950° C., 975° C., and 1,000° C., and aplurality of bodies were obtained.

Next, by baking a conductive paste for external electrodes containing Cuas a major component onto the surface of each body, external electrodeswere formed. Then, polarization was performed in a silicone oil at 80°C. at an electric field intensity of 3 kV/mm, and thereby multilayerpiezoelectric devices of Sample Nos. 1a to 9c were fabricated. The sizeof each multilayer piezoelectric device was 6 mm in length, 6 mm inwidth, and 8 mm in height.

Subsequently, an electric field with an intensity of 2 kV/mm was appliedin the form of a triangular wave with a frequency of 0.1 kHz to each ofthe resulting samples. The distortion factor in the thickness directionwas measured with an inductive probe and a differential transformer. Thepiezoelectric constant d₃₃ was calculated by dividing the distortionfactor by the electric field.

Furthermore, with respect to each of samples of Sample Nos. 1b, 2b, and5b, the contents (concentrations) of Nb₂O₅ and CuO were measured at fourpoints in the piezoelectric ceramic layer at different distances fromthe internal electrode layer. That is, although the thickness of thepiezoelectric ceramic green sheet was 120 μm as described above, thethickness of the piezoelectric ceramic layer was decreased to about 100μm due to contraction during the firing process. With respect to fourpoints in the piezoelectric ceramic, respectively, at distances of 3 μm,6 μm, 20 μm, and 50 μm from the internal electrode layer, theconcentrations (contents) of Nb₂O₅ and CuO were measured using a WDX(wavelength dispersive X-ray spectrometer).

Table 1 shows the Nb₂O₅ content on the basis of the total solid contentin the internal electrode pattern, the firing temperature, and thepiezoelectric constant d₃₃ in each sample, and Table 2 shows theconcentrations of Nb₂O₅ and CuO at each measuring point in thepiezoelectric ceramic layer.

Since the WDX cannot determine the oxidation numbers of atoms, there isa possibility that in some cases, Cu may be dispersed as Cu₂O.Consequently, strictly speaking, the CuO concentration in Table 2 showsthe total of the CuO concentration and the Cu₂O concentration.

TABLE 1 Firing Piezoelectric Sample Nb₂O₅ content temperature constantd₃₃ No. (wt %) (° C.) (pm/V) 1a* 0.0 1000 410 1b* 0.0 975 430 1c* 0.0950 440 2a 5.0 1000 530 2b 5.0 975 550 2c 5.0 950 565 3a 10.0 1000 5753b 10.0 975 595 3c 10.0 950 645 4a 15.0 1000 620 4b 15.0 975 655 4c 15.0950 695 5a 20.0 1000 655 5b 20.0 975 680 5c 20.0 950 720 6a 25.0 1000675 6b 25.0 975 715 6c 25.0 950 725 7a 30.0 1000 680 7b 30.0 975 705 7c30.0 950 745 8a 35.5 1000 680 8b 35.5 975 725 8c 35.5 950 750 9a 40.01000 — 9b 40.0 975 — 9c 40.0 950 — *Out of the range of the presentinvention

TABLE 2 Distance from Nb₂O₅ internal Nb₂O₅ CuO Sample content electrodeconcentration concentration No. (wt %) layer (μm) (wt %) (wt %) 1b* 0 35.6 0.5 6 5.7 0.3 20 5.6 0.0 50 5.6 0.0 2b 5 3 6.0 0.8 6 5.8 0.4 20 5.80.1 50 5.6 0.0 5b 20 3 6.7 0.5 6 6.1 0.2 20 5.8 0.0 50 5.7 0.0 *Out ofthe range of the present invention

As is evident from Table 1, in Sample Nos. 1a to 1c, even if the firingtemperature is varied, the piezoelectric constant d₃₃ is low at 410 to440 pm/V.

In contrast, in Sample Nos. 2a to 8c, the piezoelectric constant d₃₃ ishigh at 530 to 750 pm/V, and it has been found that the piezoelectricconstant d₃₃ improves compared with Sample Nos. 1a to 1c.

Furthermore, as is evident from Table 2, in Sample No. 1b, although theinternal electrode pattern does not contain Nb₂O₅, since thepiezoelectric ceramic composition contains the Nb component as shown incompositional formula (1), such a Nb component is substantiallyuniformly contained, as Nb₂O₅, in the piezoelectric ceramic layer.

In contrast, in Sample Nos. 2b and 5b, since Nb₂O₅ is also contained inthe internal electrode pattern, Nb₂O₅ is diffused from the internalelectrode layer into the piezoelectric layer during the firing process,and the concentration of Nb₂O₅ is distributed in the piezoelectricceramic so as to be relatively high in the vicinity of the internalelectrode layer and to decrease with distance from the internalelectrode layer.

That is, Cu in the internal electrode pattern diffuses from the internalelectrode layer into the piezoelectric ceramic layer during the firingprocess. In the case where the internal electrode pattern does notcontain Nb₂O₅, since Nb₂O₅ does not diffuse from the internal electrodelayer into the piezoelectric ceramic layer, due to the diffusion of Cu,it is not possible to improve the piezoelectric constant d₃₃.

In contrast, in the case where the internal electrode pattern containsNb₂O₅, since Nb₂O₅ diffuses from the internal electrode layer into thepiezoelectric ceramic layer as described above, the decrease in theaverage valence in the B site of the composition of the piezoelectricceramic due to the diffusion of Cu is charge-compensated for by Nb₂O₅.Thereby, low-temperature firing is enabled, and a sufficientpiezoelectric constant is ensured.

Furthermore, as is evident from comparison among samples a to c in eachof Sample Nos. 2 to 8, as the firing temperature decreases, thepiezoelectric constant d₃₃ increases, and at the firing temperature of950° C., the better piezoelectric constant d₃₃ can be obtained.

However, with respect to Sample Nos. 9a to 9c, the Nb₂O₅ content on thebasis of the total solid content in the internal electrode pattern ishigh at 40.0% by weight, and therefore, it is not possible to obtain thepiezoelectric constant d₃₃. The reason for this is that since the Cucontent decreases as the Nb₂O₅ content increases excessively, poorconnection occurs between the external electrodes and the internalelectrode layers. Consequently, it has been found that in the case of amultilayer piezoelectric device having a structure as that in thisexample, the Nb₂O₅ content on the basis of the total solid content ispreferably less than 40.0% by weight.

Furthermore, FIG. 2 is a graph showing the relationship between theNb₂O₅ content and the piezoelectric constant d₃₃. The abscissa axisrepresents the Nb₂O₅ content (wt %), and the ordinate axis representsthe piezoelectric constant d₃₃ (pm/V). The solid diamond represents thefiring temperature of 1,000° C., the solid square represents the firingtemperature of 975° C., and the solid triangle represents the firingtemperature of 950° C.

As is evident from Table 1 and FIG. 2, it has been confirmed that as theNb₂O₅ content increases and as the firing temperature decreases, thepiezoelectric constant d₃₃ tends to increase.

Example 2

Multilayer piezoelectric devices of Sample Nos. 11a to 19c werefabricated by the same method and procedure as in [Example 1] exceptthat WO₃ containing a hexavalent metal element was used, instead ofNb₂O₅, as the metal oxide incorporated in the internal electrodepattern.

Next, with respect to Sample Nos. 11a to 19c, the piezoelectric constantd₃₃ was calculated by the same method and procedure as in [Example 1].Furthermore, with respect to the samples in each of Sample Nos. 11b,12b, and 15b, the concentrations (contents) of WO₃ and CuO were measuredusing a WDX (wavelength dispersive X-ray spectrometer).

Table 3 shows the WO₃ content on the basis of the total solid content inthe internal electrode pattern, the firing temperature, and thepiezoelectric constant d₃₃ in each sample, and Table 4 shows theconcentrations of WO₃ and CuO at each measuring point in thepiezoelectric ceramic layer.

TABLE 3 Firing Piezoelectric Sample WO₃ content temperature constant d₃₃No. (wt %) (° C.) (pm/V) 11a* 0.0 1000 410 11b* 0.0 975 430 11c* 0.0 950440 12a 5.0 1000 460 12b 5.0 975 490 12c 5.0 950 565 13a 10.0 1000 52013b 10.0 975 555 13c 10.0 950 610 14a 15.0 1000 555 14b 15.0 975 595 14c15.0 950 680 15a 20.0 1000 595 15b 20.0 975 630 15c 20.0 950 715 16a25.0 1000 640 16b 25.0 975 700 16c 25.0 950 750 17a 30.0 1000 675 17b30.0 975 745 17c 30.0 950 745 18a 35.5 1000 680 18b 35.5 975 735 18c35.5 950 765 19a 40.0 1000 — 19b 40.0 975 — 19c 40.0 950 — *Out of therange of the present invention

TABLE 4 Distance from WO₃ internal WO₃ CuO Sample content electrodeconcentration concentration No. (wt %) layer (μm) (wt %) (wt %) 11b* 0 30.0 0.5 6 0.0 0.3 20 0.0 0.0 50 0.0 0.0 12b 5 3 0.2 0.5 6 0.1 0.2 20 0.10.0 50 0.1 0.0 15b 20 3 0.6 0.7 6 0.5 0.4 20 0.4 0.0 50 0.4 0.0 *Out ofthe range of the present invention

As is evident from Table 3, in Sample Nos. 11a to 11c, even if thefiring temperature is varied, the piezoelectric constant d₃₃ is low at410 to 440 pm/V.

In contrast, in Sample Nos. 12a to 18c, the piezoelectric constant d₃₃is high at 460 to 765 pm/V, and it has been found that the piezoelectricconstant d₃₃ improves compared with Sample Nos. 11a to 11c.

Furthermore, as is evident from Table 4, in Sample No. 11b, since thepiezoelectric ceramic does not contain WO₃ (refer to compositionalformula (1)) and since the internal electrode pattern also does notcontain WO₃, no WO₃ is detected.

In contrast, in Sample Nos. 12b and 15b, since WO₃ is contained in theinternal electrode pattern, WO₃ is diffused from the internal electrodelayer into the piezoelectric layer during the firing process, and theconcentration of WO₃ is distributed in the piezoelectric ceramic so asto be relatively high in the vicinity of the internal electrode layerand to decrease with distance from the internal electrode layer.

That is, Cu in the internal electrode pattern diffuses from the internalelectrode layer into the piezoelectric ceramic layer during the firingprocess. In the case where the internal electrode pattern does notcontain WO₃, since WO₃ does not diffuse from the internal electrodelayer into the piezoelectric ceramic layer, due to the diffusion of Cu,it is not possible to improve the piezoelectric constant d₃₃.

In contrast, in the case where the internal electrode pattern containsWO₃, since WO₃ diffuses from the internal electrode layer into thepiezoelectric ceramic layer as described above, the decrease in theaverage valence in the B site of the composition of the piezoelectricceramic due to the diffusion of Cu is charge-compensated for by WO₃.Thereby, low-temperature firing is enabled, and a sufficientpiezoelectric constant is ensured.

Furthermore, as is evident from comparison among samples a to c in eachof Sample Nos. 12 to 18, as in [Example 1], as the firing temperaturedecreases, the piezoelectric constant d₃₃ increases, and at the firingtemperature of 950° C., the better piezoelectric constant d₃₃ can beobtained.

However, with respect to Sample Nos. 19a to 19c, the WO₃ content on thebasis of the total solid content in the internal electrode pattern ishigh at 40.0% by weight, and therefore, it is not possible to obtain thepiezoelectric constant d₃₃ for the same reason as described in [Example1] with respect to Sample Nos. 9a to 9c.

Furthermore, FIG. 3 is a graph showing the relationship between the WO₃content and the piezoelectric constant d₃₃. The abscissa axis representsthe WO₃ content (wt %), and the ordinate axis represents thepiezoelectric constant d₃₃ (pm/V). The solid diamond represents thefiring temperature of 1,000° C., the solid square represents the firingtemperature of 975° C., and the solid triangle represents the firingtemperature of 950° C.

As is evident from Table 3 and FIG. 3, it has been confirmed that as theWO₃ content increases and as the firing temperature decreases, thepiezoelectric constant d₃₃ tends to increase.

1. A method for manufacturing a multilayer piezoelectric device, themethod comprising: alternately stacking internal electrode patterns andpiezoelectric ceramic green sheets to form a laminate; and firing thelaminate to form a body in which internal electrode layers andpiezoelectric ceramic layers are alternately stacked, wherein thepiezoelectric ceramic green sheets contain a compound oxide representedby Pb(Ti, Zr)O₃ as a major component, the internal electrode patternscontain a conductive powder containing Cu and at least one of apentavalent metal element and a hexavalent metal element, and duringfiring, the metal element is diffused in the form of a metal oxide fromthe internal electrode layers into the piezoelectric ceramic layers suchthat the concentration of the metal oxide decreases with distance fromthe internal electrode layers.
 2. The method for manufacturing amultilayer piezoelectric device according to claim 1, wherein the metalelement is at least one element selected from the group consisting ofNb, Sb, Ta, and W.
 3. The method for manufacturing a multilayerpiezoelectric device according to claim 1, wherein the metal oxide is byany one of Nb₂O₅, Sb₂O₅, Ta₂O₅, and WO₃.
 4. The method for manufacturinga multilayer piezoelectric device according to claim 1, wherein thecontent of the metal oxide is less than 40.0% by weight on the basis ofthe total content of the conductive powder and the metal oxide.
 5. Themethod for manufacturing a multilayer piezoelectric device according toclaim 1, wherein firing is conducted at a temperature of from about 950°C. to about 1000° C.
 6. The method for manufacturing a multilayerpiezoelectric device according to claim 5, wherein firing is conductedfor about 5 to 10 hours.
 7. The method for manufacturing a multilayerpiezoelectric device according to claim 1, wherein the firing isconducted in an atmosphere having an oxygen partial pressure between anequilibrium partial oxygen pressure for Pb—PbO and an equilibriumpartial oxygen pressure for Cu—CuO.